1. Industrial Field of Utilization
The present invention relates to an optical disk apparatus, which irradiates an optical disk with a laser beam, a focus control method thereof, and a computer program used for the focus control method.
2. Prior Art
Recordable optical disks such as CD-R (Compact Disc-Recordable) and CD-RW (Compact Disc-Rewritable) have been available in the prior art. When various data such as music data are recorded on these optical disks, optical disk recording/reproducing apparatuses such as CD-R or CD-RW drive units are used. These optical disk recording/reproducing apparatuses carry out information recording by irradiating a recording face formed on one side of an optical disk with laser beam from an optical pickup according to information to be recorded. On the other hand, when information recorded on the optical disk is read out, laser beam is also irradiated from the optical pickup to read the information from the return light of the laser beam reflected back from the record face of the optical disk.
When irradiating laser beam on the recording face of the optical disk, the optical disk recording apparatuses perform focus control so that the laser beam of a desirable spot diameter will be irradiated on the recording face. In this focus control operation, return light of the laser beam irradiated on the recording face of the optical disk is received, and focus control is performed using an astigmatic method.
The above-mentioned focus control in a conventional optical disk recording apparatus is feedback control in which a return light of the laser beam irradiated on an optical disk is acquired to perform the focus control based on the return light acquired. Therefore, if the laser beam is irradiated to an area with no reflecting face formed on it, a sufficient amount of return light cannot be obtained to disable the focus control.
For example, suppose that a thermo-sensitive face is provided on an optical disk on the side opposite to a recording face on which music data or the like is recorded. Suppose further that the thermo-sensitive face is irradiated with laser beam to discolor the thermo-sensitive face so that a character image such as a music title of music data will be formed. In this case, since the laser beam has to be irradiated on the thermo-sensitive face from which an amount of reflected light equal to or larger than that from the recording face cannot be obtained, accurate focus control cannot be performed.